The increasing demand for plastic containers, plus the increasing cost of materials, has encouraged the production of containers having a very thin wall thickness. These containers are extremely practical from the utility aspect, but present some problems in manufacture. A major problem in the manufacture of thin wall containers is occasioned by the introduction of foreign matter into the raw material. The smallest particle of such foreign matter will cause a perforation in the wall of a container, and therefore permit the leakage of any fluid product from the container. As the manufacture of plastic containers incorporates the removal of excess plastic (flashing) after the container is molded, and the recycling of the excess plastic, it is virtually impossible to assure that absolutely no foreign matter ever contaminates the raw material stock. This results in an occasional container being manufactured which has a perforated wall, but which, in all other aspects, appears to be an acceptable product. It is therefore imperative that all containers are checked for such conditions before subsequent processing.
Depending on the size of the container, and the equipment used for production, containers may be produced at speeds ranging from 5 to 120 parts per minute. It would be impractical, even at the slowest production rate, to inspect the containers by manual methods. The inspection must be an automatic function. One method of inspecting containers for leakage is to inject air under pressure into the container by a sealing type of probe or closure, such as is disclosed in U.S. Pat. No. 3,495,441 to Laub. The pressure is required to reach a prescribed level and, in some cases, to hold that pressure for a period of time. In either case, the attainment of the pressure is established by a pressure sensor, and the actuation of a valve or switch.
Thin wall containers are designed to carry food or non-toxic products, and not to withstand pressures in excess of those caused by the product weight. The pressure which can be used for leak detection is therefore limited, in most cases being confined to a few inches of water column. The differential in such pressures caused by leakage are minute, often not exceeding 0.2 inches of water column, and a pressure sensor which will respond to such pressure differentials is necessarily an extremely sensitive instrument. An excellent example of pressure sensing apparatus for testing such containers is disclosed in U.S. Pat. No. 4,157,656 to Walle.
However, one problem which remains in the testing of such containers is that the sensor can still be susceptible to any variations in pressure which occur in the test system, but which may not be part of the test procedure. Although variations in the test pressure can be held to a minimum by carefully controlling the initial pressure supply, this has little effect on the residual pressure factor, which is the pressure left in the system (not exhausted) between tests. At low pressures the rate of pressure decay is comparatively slow, and at fast production rates, the system can become pre-loaded with pressure before the test is initiated, requiring constant adjustment of the initial pressure. Compensating for residual air pressure in a system actually incorporates such pressure as part of the test. Therefore, any variable which permits a longer or shorter exhaust time, such as a change in conveyor speed or blank spaces on the conveyor, will negate the test until the residual pressure is reinstated, or until the system has been readjusted. This entails constant attention to the leak detection system, the rejection of many acceptable containers, and the acceptance of "leakers".